What Does Anchor Pattern (ASTM, NACE, ISO) Mean?
Anchor pattern is the term to describe the surface roughness of iron, steel or other metals consisting of craters, cracks and pores forming peaks and valleys. The roughness is usually not visible to the unaided eye and viewed in a microscope with 200x minimum magnification.
Although generally considered as the surface profile after sandblasting, anchor pattern is inherent to metal surfaces and is necessary for the proper bonding of the coating to the substrate. If the natural anchor pattern does not meet the requirement, the metal undergoes surface preparation by sand blasting, wire brushing, water blasting, chemical etching or other methods recommended by the paint or coating manufacturer.
Anchor pattern measurement is in mil or 1/1000th of an inch.
Corrosionpedia Explains Anchor Pattern (ASTM, NACE, ISO)
The anchor pattern affects not only the coating adhesion but also the coating thickness. In a coarse anchor pattern, the peaks will have thin coatings and the deep valleys will be difficult to penetrate.
Coating manufacturers usually specify the appropriate anchor patterns that should be checked before applying any coating.
There are different methods of measuring the anchor pattern or surface profile. These industry groups, National Association of Corrosion Engineers (NACE), International Organization for Standardization (ISO), and ASTM International (known as the American Society for Testing and Materials until 2001) share a “referee” method using a microscope to measure surface profile height described as the distance from the “top of the highest peak to the bottom of the lowest valley” in the microscope’s field of view. The surface profile height is the average of 20 measurements.
However, the microscope is not practical to use in the field. Instead, any of the following alternative methods are used:
ISO 8503-1 measures “the height of the major peaks relative to the major valleys” using surface profile comparators.
ASTM D7127 measures the “positive and negative vertical deviations” from a mean line using a portable stylus instrument. This method also determines the peak density or the number of peaks in a given profile area. Some coating experts believe that optimizing both the peak height and density can improve coating adhesion.
ASTM D4417-11 measures surface profile based on three different methods: a surface comparator to measure the peak-to-valley height of the surface similar to ISO 8503 – 1, a depth gage to measure the depth of profile, and the composite plastic tape that is impressed into the surface to get the reverse image of the anchor pattern.
The composite or replica tape is the most used measuring method.